The present preferred embodiment concerns a device for simultaneous double-sided printing of a recording medium, with a revolving first toner image carrier and a revolving second toner image carrier; with means for generation, on the first toner image carrier, of a first toner image comprising toner that is charged with a first polarity; with a device for generation, on the second toner image carrier, of a second toner image comprising toner that is likewise charged with the first polarity; with a charge shifting device that is suitable to shift the charge of the first toner image located on the first toner image carrier to a second polarity which is opposite to the first polarity; and with a first transfer printing point at which the recording medium is passed between the first toner image carrier and the second toner image carrier and at which an electrical field can be generated via which the first toner image shifted to the second polarity and the second toner image are separated from the first or second toner image carrier and are transferred to the side of the recording medium facing the respective toner image carrier.
The preferred embodiment also concerns a printer or copier with such a device and a method for simultaneous double-sided printing of a recording medium.
A device of the aforementioned type is, for example, known from WO 98/39691 and the parallel U.S. Pat. No. 6,246,856 B1. In this known device the recording medium is formed by a paper web and the first toner image carrier and the second toner image carrier are formed by transfer belts that are arranged essentially mirror-symmetrically relative to the paper web, above and below the same. The device for generation of the first and the second toner image is formed by transfer printing points between an associated photoconductor belt and the respective transfer belt.
The known device can be operated in two different operating modes, a multi-color print collection mode and a continuous printing mode. In multi-color print collection mode toner images are successively generated on the photoconductors in primary colors or component colors (what are known as color separations) in an electrophotographic method and are successively transfer-printed onto the respective transfer belt. The individual color components are “collected” on the transfer belt, i.e. superimposed in register, such that a first color toner image results on the first transfer belt and a second color toner image results on the second transfer belt. The first transfer belt and the second transfer belt are subsequently pivoted onto the paper web in the first transfer printing region and the first toner image and the second toner image are transfer-printed onto the top or bottom of the paper web.
Only one color is printed in the continuous printing mode, meaning that no color separations are collected on the transfer belt. Instead of this, the first toner image and the second toner image are transfer-printed from the associated photoconductor onto the respective transfer belt and directly transfer-printed onto the top or bottom of the paper web at the first transfer printing point in the course of a continuous processing.
However, the first toner image carrier and the second toner image carrier do not need to be formed by an intermediate carrier; rather, they can also be formed by photoconductors, for example, in particular photoconductor belts from which transfer printing occurs directly onto a recording medium. In the present document the term “toner image carrier” is to be understood in this generality.
Various embodiments of the first transfer printing point and the charge transfer device are described in detail and their functionality is explained in EP 1 110 125 B1 (parent application), EP 1 465 023 A1 (divisional application) and the parallel application U.S. Pat. No. 6,556,804 B1. These explanations should not be repeated here, but rather are incorporated by reference into the present specification.
As is explained in the cited documents, the toner of the first toner image and the toner of the second toner image are initially charged with the same first polarity. The toner requires this charging with the first polarity for the electrophotographic process. It is typically generated via triboelectric charging in a developer station. Before the first toner image and the second toner image can be simultaneously transfer-printed onto the opposite sides of the recording medium, one of the toner images (in the present case the first toner image) must have its charge shifted. Both toner images then experience an electrical attraction force in the direction of the recording medium in a suitable electrical field provided in the transfer printing region, and via these electrical attraction force they are transfer-printed onto the recording medium.
In this known device the print result is, however, not always satisfactory. In practice a different transfer printing efficiency often results between the first toner image carrier and the one side of the recording medium and the second toner image carrier and the other side of the recording medium. The resulting print images then deviate from one another in their optical appearance. Moreover, it can lead to an alternating influencing of the two print images. For example, when a flat pattern is printed on the one side of the recording medium and isolated characters are printed on the other side, it can occur that the isolated characters appear dimly in the flat pattern.
A device for double-sided printing of a carrier material is known from the document JP 11231597 A, in which a toner image is respectively generated on a photoconductor drum and transferred to a respective transfer belt. The transfer belts contact the carrier material on opposite sides such that the toner images are transferred from the transfer belts onto the side of the carrier material contacted by the respective transfer belt. Charge corotrons are provided that recharge the toner particles of the developed toner image on the surface of the photoconductor drum. The charge of the photoconductor drum is thereby also changed such that the toner particles no longer adhere on the surface of the photoconductor drum due to their charge but rather merely due to adhesion forces. This has the result that toner particles also arrive at regions of the photoconductor that are not to be inked (and thus alter the print image), in particular in border regions of inked surfaces.
A device for double-sided printing of a carrier material is known from the document JP 08292614 A, in which a respective toner image is generated on a photoconductor drum and is transferred onto a respective transfer belt. A recharging device is respectively provided in order to recharge the toner particles of the toner images transferred from a photoconductor drum to the respective transfer belt. The transfer belts contact a carrier material to be printed on two opposite sides, such that the toner images are transfer-printed from the transfer belts onto opposite sides of the carrier material.
An arrangement for double-sided printing of a carrier material is known from the document WO 00/14607 A, in which a respective toner image is transferred from a transfer belt onto opposite sides of a carrier material to be printed. A charge shifting corotron is thereby provided in order to shift the charge of the transfer printings of a toner image to be transferred onto the carrier material.